The present invention relates generally to the field of communication systems, and more particularly to methods of searching for the Home Public Land Mobile Network (HPLMN) in a radiotelephone system. Although the invention can be used in a wide range of applications, it is described in connection with a GSM cellular telephone.
A GSM cellular telephone or mobile station (MS) operates with a subscriber identity module (SIM) card that specifies the MS""s HPLMN. From both a user""s and a cellular service provider""s (CSP) perspective, it is desirable for the MS to be operating in the HPLMN. The user does not incur additional charges, such as roaming charges, when the MS operates in the HPLMN. The CSP earns more revenue when the MS is operating in the HPLMN.
During times when the MS is not operating in the HPLMN, the GSM ETSI specifications provide a mechanism for the MS to periodically search for the HPLMN. The mechanism utilizes a HPLMN search timer. The timeout value is configurable by the CSP and resides on the SIM card. The timeout value is set to a value in six minute increments and specifies how often the MS should search for the HPLMN. FIG. 7 is a flow chart that illustrates a known method of searching for the HPLMN. At the start of the method, the MS is camped on a non-HPLMN. In block 702, the MS is in its lowest power state. In block 704, the HPLMN search timer is set to expire in six minutes. In decision block 706, the method determines whether the timer has expired. If no, the method continues to check until the timer has expired. When the timer expires, a full search for the HPLMN is performed (block 708). A full search includes the steps of measuring the RF carrier level on every frequency supported by the MS, finding carriers of a suitable level, reading broadcast information on the suitable carriers and determining if the carriers are in the HPLMN. Next, the method determines whether the HPLMN is found. If no, the MS returns to its lowest power state. If yes, camping procedures on the HPLMN are initiated (block 710) by first searching for the strongest available channel of the HPLMN and then by registering the MS on the HPLMN.
A limitation of the above method is that the timeout value of the HPLMN search timer can be too long. Six minutes may be inadequate, in certain instances, to find the HPLMN soon after it becomes available. For example, if a user moves out of the MS""s HPLMN coverage area and quickly returns to the HPLMN coverage area, the user must wait until the HPLMN search timer expires before the MS will attempt to find the HPLMN. Another limitation of the above method is that once the HPLMN search is initiated, it can take up to two minutes to complete. The lengthy completion time can be attributed to the search consisting of taking receive signal level measurements on every channel that the MS is capable of accessing, synchronizing to suitable channels and reading broadcast data on those channels until a channel of the HPLMN is found.
Another method of finding the HPLMN allows the user to manually search for the HPLMN by navigating through a series of menus and then initiating a search for the HPLMN. Like the previously described method, this method also has a lengthy completion time. In addition, the user needs to be aware of when the HPLMN becomes available before initiating the search. If a search is initiated before the HPLMN is available, the search will prove unsuccessful and unnecessarily drain the MS""s current.
The limitations of the previously discussed methods of searching for the HPLMN could be overcome by implementing a method that continuously searches for the HPLMN. However, such an activity would cause the battery life of the MS to be significantly reduced. The process of a continuous search takes additional processor cycles and causes the internal components of the MS to be in their receive ready (higher current-drawing) state.
Therefore, there exists a need for a method of searching for the HPLMN that strikes a compromise between a continuous search that drains the resources of the MS and a long-interval search, that is too slow for desirable operation.